Method of producing large circular currents



Jan. 31, 1961 w. H. BENNETT METHOD OF PRODUCING LARGE CIRCULAR CURRENTSOriginal Filed Nov. 22, 1957 151.]. 1:25:14 25 26 I? ,26 25 27 0 0SOURCE SOURCE D G SOURCE STEADY COMPONENT PULSE INTER PULSE I INTERVALTIME WILLARD H. BENN ETT.

INVENTOR ATTORNEY berwbich illustrates the METHOD OF PRODUCING LARGECIRCULAR C URREN TS Willard H. Bennett, 174 Chesapeake St. SW.,Washington, D.C.

'(Gi'anted under Title 35, US. Code (1952), sec. 266) The inventiondescribed herein may be manufactured and used by or for the Governmentof the United States i of America for governmental purposes without thepayment of any royalties thereon or therefor.

The present invention is a continuing application of Serial No. 698,315,filed November 22, 1957, now Patent No. 2,905,842, and relates toelectron accelerating devices and more particularly to a method forproducing sustained magnetic self-focusing streams of relativisticelectrons in a closed loop.

'Herctofo're devices have been used for accelerating electricallycharged particles such as electrons to high velocity and hence a highpotential by means of magnetic induction effects. The electrons are thendiverted from their circular orbit to produce a desired result such asbombarding a target to produce X-rays. It has also been proposed that aself-focusing stream be produced in a closed loop in a betatron, whereinmotions of the electr'o'ns in a self-focusing stream in directionstransverse to the ads of the stream would produce radiation which woulddamp such motion and that such damping would oppose the thermaldispersion of the stream. Such a stream wouldinvolve the simultaneousacceleration of all of the electrons in the stream and have one majordifficulty. The combined action of the accelerating electric field andthe self-magnetic field of the current in the "stream drives theelectrons toward the axis. The continued application of the acceleratingelectric force produces a continually increasing density of electronswith their associated space-charge neutralizing ions .near the axis, andthe rate at which energy must be supplied to compensate the loss ofenergy of the electrons due to their collisions with the ions increasescorrespondingly. Such a rate may amount to thousands of kilowatts.

It is accordingly, an object of the present invention to provide amethod for producing self-focusing streams of relativistic electronsin aclosed loop.

Another object is to provide a method which is ca pable of holdingelectrons in a closed loop orbit sufiiciently long to prevent theelectrons from escaping from the guide field.

Still another object is to provide a method which is capable ofintensifying the current within a closed loop sufficientto produceself-focusing streams.

Other objects and advantages of the present invention will hereinafterbecome more fully apparent from the following description of the annexeddrawings, which illustrates the preferred embodiments, and whereini Fig.1 schematically illustrates an end view of the deviceofthe presentinvention; V H

- Fig. 2 illustrates a section 'throughthe dischargechami ber ofthedevice; I

Fig. 3' illustrates another section through the chamelectron injectiontube and-the electronpath;

Fig. 4 is another section through th e chamber'which illustrates 'softiron fmembers on the outer surface l i l "Q 2,970,273" l Prensa Jan. 31,1951 Fig. 5 represents the steady magnetic guide-field with additionalsawtooth pulses added thereto; and

Fig. 6 illustrates a modification of the device of Fig. 1.

The device of the present invention used to carry out the methodcomprises a cylindrical or pill box shaped evacuated discharge chamberpositioned within a magnetic guide-field similar in form to that of acyclotron. Electrons from an electron linear accelerator are injected inpuses at full energy into the chamber within the guide-field. Theelectrons injected in each pulse are held by an increase in theguide-field lasting long enough for the electrons to radiate energy andshrink in loop radius sufficient to prevent the electrons from escapingfrom the chamber. By successively injecting electrons into the chamber,the cycling current is built up to the minimum critical value forself-focusing and held there by the guide-field.

Now referring to the drawings wherein like reference charactersrepresent like by illustration in Fig. l, a device according to thepresent invention. The device includes similar to that of a cyclotronrate. The magnetic field contour which includes a pair 15 and 16 searated by an air Yoke member 18 completes the magnetic circuit for theflux set up in the cylindrical pole pieces. The cylindrical poe pieces15 and 16 are surrounded by an annular winding preferably split into twocoils 19 which are Wound in the same relative direction and series forenergization from a direct current source 21 to maintain the polessteadily magnetized.

produce a stabilizing her to substantially The chamber 17 material suchas g ass, sillimanite, porcelain, etc., with side walls 22 supported bya support rod 23 across thecenter of the chamberwhich prevents thechamber from collapsing when under a high vacuum. The chamber may beheld in position by any means familiar in theart, one ofwhich isillustrated by two pairs of supports 24 on the outer circumferentialsurface and adapted to be positioned within the magnetic guide-fieldwith the lines of force symmetrical with the axis of the chamber. Thecircumferential surface of the chamber is surrounded by two pairs ofcoils 25 and 26 positioned on opposite sides of holders 24 and each ofwhich are wound in the same relative directions and connected in seriesfor ehergization respectively from variable direct current sources 27and 28 which may be the same or different and which can be steeplyincreased in pulses. The pulsating current in coils25 and 26 is for thepurpose of increasing the flux inside the coils in approximately asawtooth manner as illustrated by Fig. 5. A pair of coils 31 areprovided along the side walls of the chamber and a pulsating currentfrom a source 32 is passed therethrough for the purpose of holdingelectron orbits in a plane within the chamber and'to prevent wobblingaong the axis during operation of the device. An electron linearaccelerator 33 of any well. known type which periodically injectselectrons into the chamber through an electron injection tube 34 islocated at the midplane of the chamberand arranged tangentially to theinner periphery of the chamher. The tangential arrangement permits theelectrons to be injected into the chamber suchthat on entering thechamberthe magnetic guide-field will force the electrons predeterminedcircular orbits.

into an orbit about the axis ofthechamber.

parts throughout, there is showna magnetic guide-field and which is madeto decrease in magnitude with increasing radius at a slow structure ismade up from steel" laminations or any other suitable material ofappropriate of cylindrical oole pieces gap within which a cylindrical orpill box shaped discharge chamber 17 is placed...

connected in.-

The spacing between the poles rovides a space distribution such as tofield on charged particles within a chamber 17 as to confine theparticles within said chamis made of any suitable insulating" Theelectron injection tube 34 is surrounded by a magnetic shielding tube 35which magnetically shields the beam of electrons from the guide-fielduntil the beam emerges from the injection tube and enters the magneticguide-field which crosses the chamber. The magnetic shield is made ofsoft iron or like magnetic material and consequently weakens the fieldat that point to aid in preventing the electrons in the initial loopsfrom hitting the end of the injection tube at 36 while the electrons arerotating within the chamber. The chamber is also provided with twopieces of soft iron 37 connected with the magnetic pole pieces andextending over to the center of the chamber at a point approximately 270degrees from the injection tube. These pieces of soft iron increase themagnetic field nearby and deflect the electron orbits away from theelectron injection tube. The combination of the locally increasedmagnetic field near the soft iron pieces 37 and the locally reducedmagnetic field near the magnetic shielding tube operates to set up afield which forces'the electrons in orbits toward the center of thechamber such that the electrons do not hit the end of the electroninjection tube. The field set up by the soft iron pieces 37 and themagnetic shielding tube 35 will be referred as thepiler field.

Fig. 6 illustrates a modification of the coil arrangement about thechamber 17. As shown in Fig. 6, the coils 25 and 26 are placcd on theinside of the chamber and the leads brought out through the chamber tothe current source. This modification operates in the same manner asthat of the coil-chamber arrangement of Figs. 17-4. 7 j

- In operation of the device to carry out the method of this inventionthe. cylindrical discharge chamber is positioned in the, radiallydecreasing magnetic field such that the lines of: force of the magneticguide-field are symmetrical with the axis of the chamber. The chamber isevacuated and high energy electrons from the electron linear acceleratorare. injected in pulses into the chamber at an energy large compared tothe rest-energy of an electron wherein they are directed in orbits bythe guidefield. Beginning near the end of each electron injection pulsea steeply increasing current is passed through the coils 25 and 26 andthen returned slowly to zero at the beginning of the next injectionpulse to provide a somewhat sawtooth-shaped increase in the guide-field.The field produced by the steeply increased current w'illbe referred toas the gripper field. The'steep rise in the guide-field produced by thegripper field reduces the radii of the oribits of the electronsthat'were just injected into the chamber, thus holding the electrons andmoving them away from the injector and toward the axis of the r n e V.if q j: rent i for self-focusing which can be determined from i therelation where c is the speed of light; e is the charge on the electronand y is the average energy of an'electr'on duefto momenta transverse tothe direction of the stream as seen in the laboratory system ofcoordinates.- The mean transverse energy y is due' principally to theinjected stream. As these electrons decrease in energy due to radiation,the value y increases in the ratio x /x where x is the energy of afreshly injected electronand x is the energy of the electron after atime, t. Representing the mean angular divergence at-injection as w, thecritical current i" after the electrons have decreased in energy to xfrom the value at injection of x;,, is determined by stream under theeffects of the self-magnetic field produced by the electron travel, andthe acceleration of the electrons by the self-magnetic field of thestream make the electrons radiate energy of motion transversetothedirection of the stream. This is to be distinguished from the radiationby the electrons due to their acceleration in the guide-field.

chamber. The motion of the electrons in approximately I circular. loopsresults in the electrons radiating some of their energy, and thereduction insenergy produces a corresponding reduction in loop radius.The reduction in gripper field which follows thesteep rise in current ismade slow enough for the correspondingincreasein loop radius of theelectrons to remain less than the concurrent decrease in loop radiuscaused by the radiation of electron energy. The injection of pulsesoffelectrons with subsequent application of the gripper field isconstantly repeated for continued use and operation of the device. Thecoils along the side Walls of the chamber act on the orbital travel ofthe electrons to maintain the orbits in a plane such that the electronpath does not wobble back and forth along the axis too-much. The

magnetic 'field' incombination 'with-the gripper field holds theelectrons in orbital paths about the axis of the chamber such thattheyare'held in space away from the walls of the'chamber.

The first phase build-up of current in the orbits spii'alljng inwardtoward the center of the guide-field continues until the total current,exceeds the minimum critical c tr- V The electrons are injected into thechamber at full energy and accumulate in the stream without beingfurther energized before the critical current for self-focusing isreached. An application of steady-state self-focusing stream is the useof the stream as a strongmagnetic guidefield for ions while the ions arebeing accelerated to very high energies by means with which theelectrons in the stream are not resonant. a V Ions can be injected intothe stream to run in the opposite direction around the loop. The ionscannotbe stored in the stream by any practical kind of process usingradiation like that which was used for storing electrons because theradiation rate from ions is too small. If-the ions are injected so thata part of the first few loops .lies inside the concentratedself-focusing stream, coulomb collisionsbetween ions'and electrons candeflect a few of the ions through the small angle needed to put thoseions in the stream. Those ions will stay in the stream until theirtransverse energy has become much greater than the ions which havelittle motion in the direction of the stream. This kind of ion injectioncan be maderapid enough to keep thestream filled with high energyfionsof ,the species being injected and thus prevent the ions formed byionization of residual gas from remainingin the stream and forming anyimportant part of thestream.

Obviously many modifications and variations -of the present inventionare possible in the .light of the above.

teachings. It is therefore to be understood that within the scope ofntheappended claims the invention may be said particle injection pulses andmore slowly decreasing said increased magnetic field to zero prior tothe begin ningiof each successive particle injectionpulse.

f l A method of producing large circularcurrents which comprisesapplying a'radially decreasing magnetic guide field symmetrically acrossan evacuated cylindrical chamber, injecting pulses of a narrow beam ofhigh speed electrons into said chamber near the inner surface thereof,applying a steeply increasing current through coils along the surface ofsaid chamber to increase the magnetic guide-field across said chambernear the end of each of said electron injection pulses for a period lessthan the time interval between successive electron pulses and moreslowly decreasing said increased current through said coils to zeroprior to the beginning of each successive electron injection pulse.

3. A method of producing large circular currents which comprisesapplying a magnetic guide-field symmetrically across a cylindricalchamber, injecting pulses of particles into said chamber near the innersurface thereof, delaying successive injection pulses long enough thatlosses in energy by radiation of the previously injected particles issuflicient to enable those previously injected particles to avoid beingejected from said chamber by successive increases in magneticguide-field, and increasing the magnetic field across said chamber foreach of said particle injection pulses and more slowly decreasing saidincreased magnetic field to zero prior to the beginning of eachsuccessive particle injection pulse.

4. A method of producing large circular currents which comprisesapplying a radially decreasing magnetic guidefield symmetrically acrossan evacuated cylindrical chamber, injecting pulses of a narrow beam ofhigh speed electrons into said chamber near the inner surface thereof,applying a steeply increasing current through coils along the surface ofsaid chamber to increase the magnetic G guide-field across said chambernear the end of each of said electron injection pulses for a period lessthan the time interval between successive electron pulses and moreslowly decreasing said increased current through said coils to zeroprior to the beginning of each successive electron injection pulse anddelaying subsequent injection pulses long enough such that losses inenergy by radiation of the previously injected electron pulses issufficient to enable previously injected electrons to avoid beingejected from said chamber by successive increases in the magneticguide-field across said chamber.

5. A method of producing large circular currents which comprisesapplying a magnetic guide-field symmetrically across a cylindricalchamber, injecting pulses of high energy electrons into said chambernear the inner surface thereof, directing said injected electrons intoorbital loops near the inner surface thereof, forcing the initial orbitsof said injected electrons into orbital loops of smaller radius byapplying a steeply rising magnetic field across at least a portion ofsaid chamber near the end of each of said injection pulses, decreasingsaid increased magnetic field so slowly that the loop radius ofpreviously injected electrons remain smaller than the loop radius atinjection, and continuously decreasing said increased magnetic field tozero prior to the beginning of each successive injection pulse.

References Cited in the file of this patent UNITED STATES PATENTS2,905,842 Bennett Sept. 22, 1959

